The present invention relates to a transistor that uses a superlattice in a channel region for improved mobility, and more particularly to an amorphous silicon (a-Si) thin film transistor (TFT).
In a typical a-Si TFT, the electrons have an effective (or average) drift mobility (.mu.) that seldom exceeds 0.5 cm.sup.2 /volt-sec due to traps, such as structual defects or impurities, in the conduction channel. Their effective mobility can be increased by filling these traps. One way of doing this is to use a large electrical field between the gate electrode and the conduction channel, such as at least 5.times.10.sup.6 V/cm. Such a large electric field increases the induced electron concentration in the channel, thus filling all the deep traps with electrons and increasing the fraction of electrons which are located in very shallow energy states and also above the mobility edge. In these latter states the mobility is the highest (about 10-20 cm.sup.2 /volt-sec). However, such a large field can result in breakdown of the gate insulator.